The invention is related generally to drive mechanisms for medical infusion pumps, and more particularly, to a system and method for driving the plunger of a syringe in a syringe pump.
The infusion of medical fluids, such as parenteral fluids, into the human body is accomplished in many cases by means of a syringe pump in which a syringe containing the parenteral fluid is mounted. Syringe pumps typically secure the syringe barrel in a fixed position and push or “drive” the syringe plunger into the barrel at a controlled rate to expel the parenteral fluid. A fluid administration set conducts the expelled parenteral fluid from the syringe barrel to the patient. Many syringe pumps have an elongated lead screw rotated by a motor and a screw drive mechanism such as a split nut that translates the rotational motion of the lead screw into linear motion. A syringe plunger driver is connected to the screw drive mechanism and to the syringe plunger for driving the plunger into the syringe barrel in accordance with the movement of the lead screw to expel the parenteral fluid.
Because syringes are of different sizes and are filled to different levels with infusion fluids, the extension of the plunger from the syringe barrel will differ from syringe to syringe. To accommodate such variations in the starting positions of the syringe plungers, screw drive mechanisms typically include a disengagement mechanism that the operator uses to disengage the screw drive mechanism from the lead screw threads. Once disengaged, the operator may move the plunger driver along the lead screw to the position of the extended syringe plunger, and then engage both the syringe plunger with the plunger driver and the threads of the lead screw with the screw drive mechanism at the new position. However, it is desirable that this disengagement mechanism and this plunger driver mechanism be easy to use and, preferably, be located at the same position so operation with one hand is possible.
Also, as is well known, syringes vary in size among manufacturers. Even syringes designed to contain the same quantity of fluid can vary substantially in outer dimensions of both length and diameter from manufacturer to manufacturer. In some prior pumps, only a very limited range of syringe sizes could be accommodated. They may be specified for use only with syringes of a particular manufacturer and only in a particular narrow size range. This limitation greatly restricts the usefulness of the pump. When a syringe from that manufacturer or one within the particular size range was not available, the pump could not be used and a different pump that would accommodate the syringe size had to be found, or the medical care facility would need to convince the supplier of the medical fluid to switch syringes or provide a new product line in which the desired syringes were used. Keeping health care costs as low as possible is desirable and requiring medical facilities to have different syringe pumps on hand that can accommodate different syringe sizes is less efficient and less cost effective than having a single syringe pump that can handle a wide range of syringe sizes.
Many plunger drivers include a plunger retainer having a pair of arms that engage the syringe plunger flange to retain it at the position of the plunger driver. These plunger retainer arms are biased inward towards each other to close around the flange and properly locate it in regard to a pushing surface that forms a part of the plunger driver. The pushing surface contacts the plunger flange and applies force to move the plunger into the syringe barrel to expel the syringe contents. Some plunger drivers include an anti-siphon feature on the plunger retainer, such as anti-siphon ledges on the plunger retainer arms, to prevent the plunger from moving into the barrel and emptying the syringe at a rate in excess of the programmed movement rate of the pushing surface when under a negative pressure condition downstream. It is desirable to avoid a siphoning condition as the rate of administration of a fluid from the syringe is typically prescribed for a patient and exceeding that rate may not meet the requirements of the prescription. This is particularly true in the case where the medicament is to be administered to the patient at a very low flow rate. Even a small amount of siphoning can exceed the prescribed rate.
Additionally, it has been found to be beneficial for the plunger driver, or some other device on the syringe pump, to verify that a syringe has been properly mounted in the syringe pump before the pump can be activated. If the syringe were not in the proper position and the plunger flange were to dislodge from the plunger driver during pump operation, some period of time may pass before an alarm is given, especially at low flow rates. It is also helpful for the plunger driver to be able to detect the existence of occlusions in the fluid line. Such detection can be performed by monitoring the force exerted against the plunger by the pushing surface of the plunger driver. In such systems, account should also be taken of the wide variations in shape of syringe plunger flanges with which the plunger driver engages. Many flanges are not flat but are instead rippled or otherwise deformed. Rather than being formed at a 90° angle to the plunger stem, many flanges are at other angles which may result in some difficulty unless the syringe pump has been designed to address such issues.
Performing all of the above functions for a wide range of syringe sizes and shapes would benefit medical care facilities in that only one pump would be needed. A syringe pump that is designed to handle syringes ranging from 1 cubic centimeter (“cc”) to 60 cc regardless of the manufacturer and regardless of the shape of the syringe flange would be of value to medical care facilities in that this syringe range encompasses most of the syringes in common use today.
Hence, those skilled in the art have recognized a need for a syringe plunger driver system and method that are capable of handling syringes of widely varying sizes and shapes while still presenting a relatively easy system to use to the operator of the pump. Such a system and method should provide a mechanism to align, firmly engage, detect the presence of the plunger of each of the syringes specified for the pump and detect excess pressure in the fluid line. Further, such a system and method should be capable of resisting siphoning of the syringe contents from syringes of all sizes usable in the pump. The invention fulfills these needs and others.